50 On Citric Acid. 



pernataiit liquor is iheiefore to be drawn off*, and the precipi- 

 tate preserved for use. 



When the supernatant liquor is removed, the precipitate, or 

 citrate of lime, is to be passed through a sieve, and then fre- 

 quently washed with warm water, till all the remaining mucilage 

 and other soluble impurities be entirely washed off. This will 

 be known, when the v/hole has remained at rest for some time, 

 t)y the water coming off clear and tasteless. The precipitate 

 having been thus well washed, let it be treated with diluted 

 sulphuric acid, in the following manner : For every ten pounds 

 of chalk or whiting, v/hich has been employed in the process, it 

 is necessary to take the quantity of nine pounds of the best oil 

 of %'itriol of commerce of the specific gravity of 1*84, or 1*845, 

 which is to be diluted with about seven gallons, or 56 lbs. of 

 v/aterf. This diluted sulphuric acid is to be gradually poured 

 upon the citrate of lime J, and the whole mixture well stirred, 

 for a considerable time, with a strong wooden spatula, that there 

 may be no chance of any of the citrate remaining unbroken, and 

 consequently unacted upon by the mineral acid. The more ef- 

 fectually to guard against this, it is advisable, just before the last 

 portions of oil of vitriol are added, to pass the whole mixture a 

 second time through the sieve : otherwise, notwithstanding the 

 utmost care, some lumps of citrate of lime may still remain im- 

 penetrated by the sulphuric acid, which would occasion a loss, 

 and probably deception. 



In conducting this part of the process, I have noticed an ap- 

 pearance worth mentioning to the practical chemist. It being- 

 necessary to stir up the mass very frequently during the gradual 

 addition of the sulphuric acid, I have observed that whenever 

 the citrate of lime became nearly all decomposed, and the earth 

 mostly saturated with sulphuric acid, the sulphate of lime preci- 

 pitates quicker and quicker after every addition of this acid, and 

 tiiat the supernatant liquor considerably increases in quantity. 

 The latter circumstance is occasioned by the increased density 

 of the precipitate ; and both appearances should always be 

 looked for, as they afford a good criterion for judging of the 

 completion of the operation. It is however proper to remark, 



• For this purpose some use plugs at difFerentTieights in the vat ; but a syphon 

 with a stop-cock is better, as with it the Ht[uor may be drawn off perfectly close, 

 and witliout any danger of accident. 



f Proust employed diluted sulphuric acid of \-\50 sp. grav. But on looking 

 to niy table of the sp. grav. of diluted sulphuric acid, (see Phil. Mag. Sept. 

 18I'2.) I see that four parts water will reduce the ' strongest sulphuric acid of 

 comjnerce to that sp grav.; and I have found that thi» is far short of the dilu- 

 tion necessary to produce the best result, 



; Some apply heat iu this case, but it is superfluous, as that evolved by the 

 mixture of water and sulphuric acid is quite sufficient to decompose the citrate. 



that 



